GH secretion is known to be inhibited by the hypothalamic hormone somatostatin (SS) and stimulated by GH-releasing hormone (GHRH) in all mammalian species studied including humans. In man, GH is released from the anterior pituitary somatotrophs in pulsatile secretory bursts occurring about 4-8 times in each 24 hour period (Devesa, J., et al., Trends Endocrinol Metab. 3:175-183 [1992] and Mason, W. T., et al., Acta Paediatr Suppl 388:84-92 [1993]). This episodic release pattern seems to be optimal for inducing the physiological effects of GH since many target tissues appear to be more sensitive to the frequency than the total amount of GH arriving at the target tissue (Robinson and Clark Growth Hormone: Basic and Clinical Aspects Isaksson, Binder, Hall and Hokfelt eds., Amsterdam, p109-127 [1987]). It is believed the episodic secretion of GH is caused by the rhythmic alternate release of the excitatory 44-amino acid peptide GHRH and the inhibitory tetradecapeptide SS, regulated through the “pituitary-hypothalamus axis” (see FIG. 1). Secreted GH, in turn, both directly and indirectly through IGF-1 appears to maintain this rhythm by stimulating SS and inhibiting GHRH release. Other neurotransmitters also modulate GH release usually by stimulating or inhibiting SS release. Additionally, other factors including exercise, sleep, glucocorticoids, thyroid hormones (e.g. TSH), sex steroids (e.g. testosterone and 17-β estradiol), free fatty acids, amino acids (e.g. arginine and ornithine), and glucose levels further modulate GH release.
In addition to the two primary endogenous regulators of GH release, SS and GHRH, a number of other peptidyl/nonpeptidyl compounds have been shown to stimulate GH release primarily through the pituitary-hypothalamus axis. These include the peptides galanin, pituitary adenylate cyclase-activation peptide (PACAP), delta sleep-inducing peptide (DSIP), and angiotensin II. These peptides, however, generally lack specificity for GH release. A number of structurally diverse nonpeptidyl GH secretagogues (e.g. Talipexole and Clonidine) are reported to stimulate GH release in vitro and in vivo, but these compounds are believed to mediate their effect through cholinergic, adrenergic, dopaminergic or serotonergic pathways and thus also lack GH releasing specificity.
Apart from GHRH, the GH secretagogues having the greatest GH releasing specificity and thus having the greatest therapeutic potential are the growth hormone releasing peptides/peptidomimetics (GHRP's) (Bowers, J. Pediatr. Endocrinol. 6:21-31 [1993]; and Schoen et al., Annual Reports in Medicinal Chemistry, 28:177-186 [1993]). These compounds can activate the pituitary-hypothalamus axis (Dickson et al., Neuroscience 53:303-306 [1993]) and act directly on the pituitary somatotroph (see FIG. 1) by an independent (non-GHRH, non-opiate and non-SS) secretory pathway. Compounds of this class can therefore be characterized by their independent GH releasing pathway. For example, somatotroph cells maximally stimulated with GHRP's can release additional GH when treated with GHRH and vice versa. Similarly the inhibitory effects of specific antagonists to GHRH or GHRP's have no effect on stimulation of GH release by the opposite secretagogue in vitro. These compounds also exhibit dose-dependent desensitization or attenuation of GH release after continuous exposure with the same or different compounds of the GHRP class. Furthermore, structurally related biologically inactive cognate GHRP compounds capable of inhibiting GH release of a particular GHRP have no effect on GHRH agonism. These effects support the independent pathway model and serve as experimental criteria for compounds belonging to the GHRP class. Surprisingly, while the GHRH receptor has been cloned in a number of species including man (Gaylin et al., Mol. Endo. 7:77-84 [1993], the GHRP receptor has remained elusive.
The paradigm compounds of the GHRP class are the synthetic methionine-enkephalin derived GHRP's identified by Bowers et al., Endocrinology 106:663-667 (1980) and Momany et al., Endocrinology 108:31-39 (1981). The most widely studied GHRP is referred to as “GHRP-6” (Momany et al., Endocrinology 114:1531-1536 [1984]; and Bowers et al., Endocrinology 114:1537-1545 [1984]) which has been shown; to be specific for GH release, has no reported long term toxicity, is well tolerated, and can elevate serum GH in a dose-dependent manner in normal humans (Bowers, J. Pediatr. Endocrinol. 6:21-31 [1993]). GHRP-6 is active in a dose-dependent manner when administered either iv, intranasally or orally, though it is poorly absorbed orally (˜0.3%). More potent second generation hepta- and hexapeptides, “GHRP-1” and “GHRP-2” (also known as KP 102), of this class have been described more recently, though these compounds are also expected to be poorly absorbed orally.